1. Field of the Invention
The present invention relates to an expansion anchor including an anchor rod having a head widening in an insertion direction, and a sleeve having a plurality of expansion tabs separated from each other by slots and extending from a plastic hinge toward a free front end of the sleeve, with the expansion tabs expanding radially upon relative displacement of the sleeve and the anchor rod.
2. Description of the Prior Art
Expansion anchors of the above-described type are inserted in preliminary formed bores provided in structural components and are anchored there by radial expansion of the expansion tabs. The expansion anchors are secured in the structural component by an expansion force which is applied by the expansion tabs to the wall of respective bores. When so-called undercut systems are used, the anchoring is provided by a form-locking connection which the radially expanded expansion tabs form with an undercut provided in the bore. Undercut systems are subdivided into conventional systems in which after a bore is formed, an undercut is formed in a separate step with a special tool, and into self-cutting systems in which undercut is formed in the bore wall upon a radial expansion of the expansion tabs. To this end, in the self-cutting systems, the expansion tabs are provided with cutters which mill or chisel the bore wall of a structural component.
In the known systems, the expansion tabs have substantially cylindrical outer surfaces. This initial geometry of the expansion tabs corresponds to the geometry of the undercut, which is usually cone-shaped, only to a very small extent. The abutting surfaces of the expansion tabs and of the undercut have curvatures deviating from each other in both circumferential and axial directions of the expansion tabs. Therefore, the application of a load to a structural component is not uniform. During application of forces to the structural component, in some regions, very high surface pressures occur which may lead to destruction of the structural component and may result in the reduction of the load value. Further, the geometry of the expansion tabs and the anchor rod head also often do not correspond to each other in the conventional systems. In many cases, the head has a conical or trumpet-shaped outer surface, while the expansion tabs, which are defined by slots formed in the cylindrical sleeve, have a cylindrical surface. It is immediately clear, that under these conditions, the curvature radii of the abutting surfaces deviate from each other. The occurring linear contacts, which are caused by different curvature radii, result in cold welding which prevents a secondary expansion in a torn structural component in an opening bore.
Accordingly, an object of the present invention is to eliminate the drawbacks of the known expansion anchor, in particular, that used in undercut systems. When an anchor is loaded, the load should be introduced into the structural component gently to prevent any destruction. Cold welding between the expansion tabs and the anchor rod head need be prevented. The rigidity of the system expansion anchor-structural component should be improved.